Tunnel Riding( Races of Stone, p. 145)
You are particularly adept at maneuvering mounts through tight spaces and underground passages.
You and your mount do not take penalties on attack rolls or to Armor Class when squeezing into or through tight spaces. You can fight in any space large enough for the mount to squeeze through. See page 148 of the Player's Handbook for more information on squeezing through tight spaces. Your having this feat does not allow a mount to squeeze through a space it would not normally be able to pass through.